Balanced line signals are pairs of analog signals of equal amplitude (approx. one volt maximum) but of opposite polarity, which are transmitted through cables and received by circuitry that amplifies the difference between the two signals. Any common mode components in the signal pair, which may be obtained both from radiated noise picked up by the cable and from hum due to ground loops between interconnected elements of the system, are cancelled out and rejected by the receiving differential amplifier circuitry, producing an amplified output lacking most of the common input noise. Thus, balanced line circuits are better able to discriminate signal from noise, resulting in improved transmission quality.
Amplifiers for use with balanced lines should be able to amplify two input signals of opposing polarity, i.e. balanced input signals, and produce balanced outputs. In addition, such amplifiers should be able to create balanced output signals from an unbalanced signal source, where needed, such as at the transmitting end of balanced lines in some systems. These amplifiers should not only reject as much of the common mode component in the input signals as possible, but should also contribute as little of their own distortion or noise as possible to the balanced outputs.
Examples of several differential amplifiers of the prior art are illustrated in FIGS. 5-8. FIG. 5 shows independent amplification of balanced input signals +IN and -IN by two separate gain stages 10 and 11. Although this circuit produces balanced output signals +OUT and -OUT when the input signals are balanced, it will not produce a balanced output from an unbalanced source. Also, each gain stage amplifies not only the desired signal, but also amplifies the incoming noise. Not only do the independent gain stages not remove common mode noise components from the incoming signal, but they also add their own distortion and noise to the outputs.
FIG. 6 shows a differential amplifier stage 12 with an inverting amplifier 13 attached to its output. The differential amplifier accepts either balanced or unbalanced inputs +IN and -IN and amplifies the differential signal while rejecting the common mode noise. The output +OUT of this stage, while lacking most of the input noise, will now carry the distortion and noise contributed by the amplifier stage 12. The inverting amplifier 13, preferably of negative unity gain, provides an output signal -OUT of opposite polarity from the first output +OUT. The distortion and noise of the first output +OUT is carried through inverted to the second output -OUT and additional distortion and noise is contributed by the inverting amplifier 13.
FIG. 7 shows two gain stages 14 and 15 coupled together by a resistor 16 across their respective negative feedback terminals. The amplifier accepts both balanced and unbalanced inputs and produces a balanced output. It does not reject the common mode signal, but neither does it amplify it. The distortion and noise contributions of both amplifiers will show in both outputs with opposite polarity so that they will not be removed differentially at the load.
FIG. 8 shows a first amplifier 17 amplifying an input signal IN and providing an output signal OUT(A) that includes the input noise amplified plus the distortion and noise added by the amplifier 17 itself. The negative feedback junction of the amplifier 17 is held at ground potential plus the amplifier contributed error at the output divided by the loop gain. At this location we see the errors of the amplifier 17 separated from the input signal. A second amplifier 18 amplifies this error and passes it along on the balanced line OUT(B) in common with the original error on the output line OUT(A). The distortion and noise contributed by the first amplifier is thus removed at the load R.sub.L by common mode rejection. However, there is no correction for any distortion and noise contributed to the balanced line OUT(B) by the second amplifier 18. The noise from the second amplifier 18 will be comparable to the noise from the first amplifier 17, and thus an improvement is only obtained if the distortion by the second amplifier 18 is less than that of the first amplifier 17. This reduces the performance of the two amplifiers operating in tandem to mostly that of the second amplifier. Fortunately, the second amplifier 18 operates under more ideal conditions than the first amplifier 16, since it only needs to swing the voltage of the first amplifier's distortion and noise contribution and not the much larger voltage of the input signal IN. Accordingly, it can often be made of very high quality, i.e. with very low distortion. For power amplification applications, in which the second amplifier 18 needs to be able to sink the high current through the load R.sub.L, it can be biased in class A mode to reduce distortion caused by this sinking of current. Other error correction techniques similarly reduce the performance of an amplifier system to that of an amplifier stage operating under more ideal conditions than another. U.S. Pat. Nos. 3,970,953 to Walker and 4,107,619 to Pass show two examples of such amplifiers.
An object of the invention is to provide a high fidelity amplifier that accepts both balanced and unbalanced input signals, amplifies the input signal with as little added distortion and noise as possible, rejects common mode noise components of the input signal, and produces a balanced differential output signal.